1. Field of the Invention
This invention relates to the fields of ethylene vinyl ester interpolymers in particulate form and to processess for alcoholyzing such interpolymers to provide vinyl alcohol-containing interpolymers.
2. Description of the Prior Art
Ethylene-vinyl ester interpolymer powders have a wide variety of applications such as flexibilizers and impact modifiers for other resins, notably, the polyvinyl chlorides, as substrate polymers for graft copolymerization with other monomers and as starting materials for the manufacture of ethylene-vinyl ester-vinyl alcohol terpolymers and ethylene-vinyl alcohol copolymers, especially by alcoholysis. Frequently, an ethylene-vinyl ester interpolymer powder is obtained by the known technique of melt dispersion, i.e., by adding the interpolymer to water maintained at a temperature above the melting point of the resin, adding a suitable surfactant or emulsifier to the molten resin mixture as a dispersing agent, mechanically stirring the mixture to obtain an acceptable dispersion of interpolymer, cooling the aqueous dispersion to at least the solidification point of the molten dispersed interpolymer particles, filtering the solidified particles from the aqueous dispersion medium, and washing and drying the resin particles.
It is known from U.S. Pat. Nos. 3,780,004 to Hoyt et al. and 3,510,463 to Bristol, and British Pat. No. 1,095,204, that particulate ethylene-vinyl ester interpolymers of relatively low vinyl ester content can be subjected to solid state alcoholysis. However, it has been observed that with finely divided ethylene-vinyl ester interpolymers containing about 30% or more vinyl ester content by weight, rapid filtration is virtually impossible and the particles comprising the resulting filter cake will, upon drying, agglomerate thereby excluding the use of the resin as a practical source material for the above-noted applications of ethylene-vinyl ester copolymers. Were it not for the tendency of ethylene-vinyl ester interpolymer particles to coalesce, a tendency which becomes more pronounced with increasing vinyl ester content and diminishing particle size the direct use of the high surface area particulate filter cake for conversion to particulate vinyl alcohol-containing polymers by solid phase alcoholysis employing caustic alcohol would be economically and technically compelling.
There is consequently a need for an effective procedure for rapidly filtering finely divided ethylenevinyl ester interpolymer particles containing 30% or more vinyl ester content by weight from aqueous dispersion media to provide a filter cake suitable for applications requiring a substantially aggregate-free powder.